Such a type of cup is used in some analysis methods, particularly for rapid diagnosis by identification, or determination of sensitivity to antibiotics (antibiogram) for a given bacterial strain. These methods are already used in a standardized way in some laboratories, with the help of thermoformed, synthetic rectangular plates, exhibiting an elongated shape and each provided with one or two rows of small tapered cups with a flat bottom constituting miniaturized test tubes. Each of the cups is initially filled either with a different dehydrated growth substrate associated with a suitable chemical indicator of consumption of the substrate or with the same dehydrated antibiotic in different concentrations or a different dehydrated antibiotic.
These methods then involve a seeding or inoculation operation by introduction of a given volume of aqueous suspension of a microbial strain in each cup. Following this operation, the cups are put into an oven for a growth phase which lasts twenty four hours.
Within the context of a bacterial identification, the consumption of the substrate belonging to each cup by the strain is displayed by the chemical indicator which changes the coloring of the culture medium contained in the cup. Reading the colorings of the different cups is performed visually and preferably automatically by a photometer equipped with a filter changer measuring the light transmitted by the suspension. The use by the unknown strain of the different substrates of the cups of the plate makes possible the biochemical identification of the strain.
Within the context of determination of the sensitivity of a given strain to antibiotics, the results are read by a photometer measuring, by turbidity, the ratio of the diffused light to the light transmitted by the suspension. The turbidity is maximum, when the strain considered is insensitive to the antibiotic tested and develops normally in the culture medium.
The photometers used in both cases comprise a light source located above the cup concerned emitting an incident light beam perpendicular to the flat bottom of the cup and three receiving diodes one of which, measuring the transmitted light, is placed directly below the flat bottom, the other two diodes measuring the diffused light and each offset about 30.degree. relative to the one measuring the transmitted light are placed opposite the wall of the tapered cup considered.
In a first embodiment now known, the analysis cups exhibit a flat bottom connected to a lateral wall giving them a tapered configuration. In regard to their dimensions, they have a considerable base diameter and a slight height. The volume of suspension contained in each of them is about 135 microliters and the necessary incubation period in the oven is twenty four hours.
This considerable period is harmful to the economical profitability of this type of microbiological analysis.
With this taken into consideration, it was imagined in a known second embodiment to reduce the diameter of these tapered cups. Thus, the optical path traveled by the incident light ray of the photometer through the bacterial suspension, for the same volume of 135 microliters, is longer, which has the effect of increasing the sensitivity of the reading photometer and therefore of making possible reliable optical measurements on bacterial suspensions with a slight growth period, more precisely four hours, for which the indication of the considered reactions is still only incipient.
However, it should be noted that the volume of bacterial suspension contained in each of these cups remains high, on the order of 135 microliters. Now, identification racks of strains and antigram can require up to thirty two cups, which is equivalent to a considerable volume of bacterial suspension (about six milliliters with the dead volume) and therefore to a high number of colonies necessary to obtain the suspension.
Further, it is known that during the microbiological analysis, the number of isolated bacterial colonies to be sampled should be as small as possible. This is due, on the one hand, to the limited number of colonies available for a given germ and, on the one hand, to the risk of bacterial mixing during the sampling. Still the level of germs should be sufficient to obtain a standard opacity of the suspensions, essential to guarantee precise results for the reading.
It is clear that considerable volumes of bacterial suspension per cup are completely incompatible with the constraints of practice.
With this taken into account, it therefore was necessary to develop an analysis cup containing a small volume of bacterial suspension and offering a relatively long optical path to the incident light ray of the photometer to reduce the period of the incubation phase to four hours.
The technical problem then encountered was linked to the surface tension phenomena of the suspension. Actually, a very small volume of suspension introduced into a cup is in the form of a droplet whose cohesion is very great and which, consequently exactly fits, with difficulty, the shape defined by the wall and bottom of known tapered cups. Consequently, the droplet can be fixed in different positions on the wall of the cup because of slight wettability of the latter.
Its meniscus therefore is practically never approximately in a plane parallel to the bottom of the cup. Further, even with the most favorable hypothesis of positioning of the meniscus, the latter is unstable and constantly evolves from a concave shape to a convex shape and vice versa.
Thus, the results obtained during colorimetric or turbidimetric reading through such cups are inexact and false. Further, the reproducibility of the results obtained is absolutely not assured. To this is added the fact that the variability of the shape of the meniscus from one cup to the next runs the risk of falsifying the overall result of the analysis.